In the medical practices of early days, a used syringe would be thoroughly sterilized for repeated use later. Any incomplete sterilization would dangerously cause a second time infection of a patient and/or any other people. To avoid such second time infection, disposable syringes are largely produced. However, large amount of discarded disposable syringes also bring us new problems of environmental pollution and safety in use. This is because no specific measures have been taken in disposing needles of the discarded syringes and exposed needles tend to easily stab nursing or cleaning personnel and result in even more infected people. To prevent discarded syringes and/or needles from injuring and infecting other people, various kinds of syringes with means for destroying used needles are developed. A typical syringe with such needle destroying means usually includes a plunger having a projected retaining hook to engage with a rear end of a needle holder, so that the needle holder and the used needle could be pulled into a barrel of the syringe. The needle being pulled into the barrel is bent and thereby destroyed. A problem with such syringe is, when the retaining hook on the plunger is about to engage with the rear end of the needle holder, there is still some medical liquid remained between the plunger and the barrel, and when the retaining hook gets in contact with the rear end of the needle holder, an air-tight space is formed in the barrel that prevents the retaining hook on the plunger from successfully engaging with the rear end of the needle holder. Clinically, a nursing personnel must first pull the needle out of the patient's body and then pushes the plunger further to engage the retaining hook with the rear end of the needle holder. At this point, medical liquid and/or patient's blood that presents in the barrel and/or the needle would jet out from the needle when the plunger is pushed forward. And, the jetted blood forms another form of contamination endangering other people.